The present invention relates to a method for the manufacture of a translucent film of good water permeability having dissociating power with respect to metallic ions and the translucent film manufactured by this method. More particularly, the present invention relates to a method for manufacturing translucent film of good water permeability, comprising a region extending inward from the face thereof of highly acetylated cellulose, a region extending inward from the back thereof of cellulose, and an intermediate region of acetylated cellulose the degree of acetylation of which decreases in the direction of from face to back. The dissociating power with respect to metallic ions of the translucent film derives from said highly acetylated region and the cellulose region serves as a supporting base and gives characteristic strength to the product.
As well-known to the art, acetylated cellulose possesses selectivity toward metallic ions. However, it is also known that homogenously acetylated film is of extremely low water permeability and unsuitable for practical application.
Attempts have been made to obtain higher water permeability. For example, an acetylated cellulose translucent film manufactured by the method disclosed in the Japanese Patent Application No. 42- 2818(1967) is known. This method is one wherein a cellulose derivative and water soluble liquid amide dissolved in an organic solvent, the solution thus obtained is cast into the form of a film, a portion of the organic solvent is evaporated for a specified period, the film is immersed in water and thereafter heated to reduce its porosity.
As apparent from the above manufacturing method, the film is composed of a thin top layer which has dissociating power and obtained by evaporating a part of the organic solvent for a specified period and a porous support is obtained by immersion in water. This method is aimed at obtaining higher water permeability by making the top layer with dissociating power as thin as possible.
However, the translucent film thus produced has various drawbacks. First, the support upon which the translucent film relies for mechanical strength is porous so that the film as a whole has extremely low strength.
Second, in the metallic ion dissociation process it is generally necessary to apply a pressure higher than ordinary osmotic pressure and so in use the pores of the support have a tendency to be pressed shut causing a reduction in the water flow rate during extended service.
Third, the top layer and the support are essentially of different structure, i.e., the former is a homogenous film and the latter is a porous film.
Thus, the films cannot be dried and must ordinarily be stored in the wet condition. Fourth, since the translucent film is formed by casting, there is a danger that foreign substances present in the cast solution will result in undesirable pinholes which reduce the dissociating power of the film with respect to metallic ions.
Also known to the art is a method wherein an extremely thin acetylated cellulose film is directly cast over a porous film in an attempt to obtain higher water permeability.
However, the same problems mentioned above remain since the support is a porous film.
One object of the present invention is to provide a method for manufacturing a translucent film of great strength, good dissociating power with respect to metallic ions and good water permeability.
Another object of the present invention is to provide a translucent film of great strength, good dissociating power with respect to metallic ions and good water permeability.